Electrolysis of water is key technology, not only for clean energy production, but to ensure a continued supply of hydrogen beyond fossil resources, essential to the manufacture of many chemical goods other than fuels. Cobalt nanomaterials have been widely identified as a promising candidate for the anode (oxygen evolution) reaction in this process, but much work has focused on applied materials or electrode design. Given the importance of oxidation state changes Co(III) → Co(IV) in the accepted reaction mechanism, in this work we look at size effects in small (4-10 nm) cobalt nanoparticles, where the ease of oxidation for lower cobalt oxidation states is known to change with particle size. To discriminate between geometric and chemical effects we have compared the catalysts in this study to others in the literature by turnover frequency (widely used in other areas of catalysis), in addition to the more commonly employed performance metric of the overpotential required to produce a current density of 10 mA cm-2. Comparisons are drawn to key examples of using well defined nanomaterials (where the surface are of cobalt sites can be estimated). This has enabled an estimated intrinsic turnover rate of ~ 1 O2 molecule per surface Co atom per second at an overpotential of 500 mV in the oxygen evolution reaction under typical alkaline reaction conditions (pH 14.0) to be identified.
Keywords: Cobalt; Nanoparticles; Oxygen evolution reaction; Particle size; Water splitting.